.beta.-isophorone has great economic significance since it is an important synthetic structural element for the production of carotinoids, vitamins and pharmaceutical products. In particular, .beta.-isophorone is required as a precursor for ketoisophorone(2,6,6-trimethylcyclohex-2-ene-1,4-dione) and trimethylhydroquinone and therewith for the production of vitamin E. In addition, it is pivotably used in syntheses for odorous substances and natural compounds such as astaxanthine and abscisic acid and derivatives.
The production of isophorone is carried out by means of acetone trimerization under condensation of the C.sub.3 structural elements. The primarily formed isomer is .alpha.-isophorone since it has, in contrast to the .beta. isomer, a double bond conjugated to the keto function. For this reason the thermodynamic equilibrium is on the side of the .alpha.-isophorone; the .beta. concentration is only approximately 1-2% and the adjustment of equilibrium takes place very slowly.
Although there are basically two different preparations for arriving at ketoisophorone, namely, the direct oxidation of .alpha.-isophorone (.alpha.-IP).fwdarw.ketoisophorone (KIP) and the indirect route via the isomerization .alpha.-isophorone.fwdarw..beta.-isophorone (.beta.-IP) in a primary step and subsequent oxidation of the .beta.-isophorone.fwdarw.ketoisophorone, the latter process is clearly advantageous. Scheme 1 presents these considerations for ketoisophorone synthesis in a clear manner. ##STR1##
Numerous methods for the isomerization of .alpha.-IP have been described in the course of time which, however, have significant disadvantages. Viewpoints such as consumption of chemicals, poor space/time yields and problems in the workup have prevented, up to the present, a practical processing reaction on a rather large scale.
A distinction between gaseous phase reactions and liquid phase reactions can be drawn in the production methods for .beta.-IP from .alpha.-IP.
Basically, four parallel reactions of .alpha.-isophorone are possible in the gaseous phase which compete with one another and can be used to a varying degree as a function of the selected temperature range and of the nature of the surface of the catalyst used.
Isophorone can react in the following manner on contact in the gaseous phase:
a.) Isomerization to .beta.-isophorone PA0 b.) Reduction to trimethylcyclohexadienes (hydrogen required for this is supplied by isophorone (IP) decomposition accompanied by carbonization phenomena) PA0 c.) .beta.-Elimination of methane to 3,5-xylenol PA0 d.) Production of mesitylene. PA0 D1=A. Heymes et al., Recherches 1971, 18, 104 PA0 D2=FR-A-1,446,246 PA0 D3=DE-OS-24 57 157 PA0 D4=U.S. Pat. No. 4,005,145 PA0 D5=EP-A-0,312,735 PA0 D6=JP 87-33019 corresp. to HEI-1-175954 of Jul. 12, 1989.
The following scheme 2 shows the reactions of .alpha.-IP catalyzed upon heterogeneous contact in the gaseous phase: ##STR2##
EP 0,488,045 B1 discloses an isomerization method in the gaseous phase (300-450.degree. C.) with a heterogeneous catalyst. Oxides and mixed oxides of Mg (group IIa), Al (IIIa), Si (IVa) and Ni (VIII) are used as catalysts which are active per se or are applied on a .gamma.-aluminum oxide carrier (specific surface 1-50 m.sup.2 /g). 1-10 kg .alpha.-IP are used per liter catalyst, the concentration of the intermediately obtained solution is about 9% .beta.-IP, the final concentration after distillation in a vacuum is 97% .beta.-IP. The granulation of NiO takes place with 1% Luviskol K90 (-polyvinylpyrrolidone). This result corresponds, relative to the amount of catalyst used and the time, to a yield of Y=0.308 kg .beta.-IP per liter catalyst and hour. A disadvantage to this is the fact that only a 9% .beta.-IP mixture accumulates per hour. The space-time (S-T) yield Y.sub.S-T =0.09 1.sub..beta..sbsb.-IP /l.sub.solution relative to the educt volume used (example 1).
In addition, the rate of removal is low, which makes the method not very attractive on an industrial scale.
L. F. Korzhova, Y. V. Churkin and K. M Vaisberg, Petrol. Chem. Vol. 31, 1991, 678 describe the reaction of .alpha.-IP at 300-800.degree. C. in the presence of heterogeneous catalysts. .gamma.-Aluminum oxide, magnesium oxide and quartz are considered as catalytic systems. The product spectrum is observed as a function of temperature and of catalyst. The formation of .beta.-IP, trimethylcyclohexadiene, 3,5-xylenol and of mesitylene are compared with each other (see scheme 2: Paths a., b., c., d.). Thus, the thermal reaction of .alpha.-IP at above 550.degree. C. on a slightly developed catalytic surface (quartz) results in a mixture of the composition c&gt;&gt;a&gt;&gt;d and b=0. The reaction of the MgO contact at 400.degree. C. shows a similar product distribution at distinctly lower temperature, namely, c&gt;&gt;a&gt;d&gt;b. The reaction takes place at 300.degree. C. in the presence of an aluminum oxide catalyst with pronounced basic-acidic surface structure with a distinct preference given to the cyclohexadiene products, namely, b&gt;&gt;c&gt;d.
On the whole, it can be assumed that a catalytic gaseous-phase isomerization is absolutely disadvantageous in several ways: It can be stated in general that these methods are disadvantageous because either the product formation is accompanied by a considerable accumulation of byproducts or the space-time yield (absolute .beta.-IP production/hkg.sub.cat) is too low.
A number of publications also relate to the isomerization in the liquid phase. The recent state of the art is represented by the following publications:
D1 discloses the isomerization of .alpha.-IP to .beta.-IP with stoichiometric amounts of MeMgX (Me=methyl; X=halogen) Grignard compound. 73% .beta.-IP is obtained under the evolution of methane in the presence of catalytic amounts of FeCl.sub.3. Mechanistic notions start with the assumption that the Grignard compound reacts as a base and does not function as the carrier of a carbanion. Excess Mg results in the production of dimer mixtures which proceed from a reductive metallic dimerization. However, the reaction of .alpha.-isophorone with molar amounts of methylmagnesium iodide in the presence of catalytic amounts of FeCl.sub.3, subsequent hydrolysis and workup by distillation is just as complicated as it is in the consumption of chemicals.
D2 relates to the isomerization of .alpha.-IP to .beta.-IP in the presence of catalytic amounts of p-toluene sulfonic acid and generally aromatic sulfonic acids, especially aniline sulfonic acid. The amount of the catalyst used is 0.1-0.2% relative to the (.alpha.-IP used. However, a low degree of conversion and a high accumulation of byproducts prevent an industrial application of the method of D2.
According to D3, the preparation of .beta.-IP takes place by means of boiling .alpha.-IP for several hours in triethanol amine, fractionation, washing the distillate with tartaric acid and sodium chloride solution. The consumption of chemicals is also considerable here.
In D4, acids with a pK=2-5 and a higher boiling point than .beta.-IP (boiling point .beta.-IP=186.degree. C./760 mm Hg) are used as catalyst. According to the patent claim the following are explicitly protected in the liquid phase:
Aliphatic and aromatic amino acids, adipic acid, p-methylbenzoic acid, 4-nitro-m-methylbenzoic acid, 4-hydroxybenzoic acid, 3,4,5-trimethoxybenzoic acid, vanillic acid, 4-trifluoromethylbenzoic acid, 3-hydroxy-4-nitrobenzoic acid and cyclohexane carboxylic acid and derivatives. The amount of catalyst used is 0.1-20 molar percent. The yield of .beta.-IP (relative to .alpha.-IP used) is 74.5%. This corresponds under the given conditions converted to the amount of catalyst used and time to a yield of Y=0.218 liters .beta.-IP per kilogram catalyst and hour.
The homogeneous catalytic isomerization of .alpha.-IP.fwdarw..beta.-IP with slightly dissociated acids represents an improvement as concerns the consumption of chemicals with .beta.-IP being continuously removed from the equilibrium. With so low a rate of removal as 11 ml/h .beta.-IP from an educt volume of approximately 0.5 kg .alpha.-IP, the space-time yield and the production of .beta.-IP with Y=0.24 kg .beta.-IP/kg.sub.cat are too low to be used in technological applications.
A similar principle is followed in D5. Acetyl acetonates of transitional metals are used as .pi. bond isomerization catalysts. Even Al (acac) displays catalytic activity. The use of the catalyst takes place in 0.01-10% by weight. Metallic catalysts of the groups IVb (Ti/Zr/Hf), Vb (V/Nb/Ta), VIb (Cr, Mo, W), VIIb (Mn/Tc/Re), of the entire group VIII and aluminum are patented. The primarily accumulating distillate has a .beta.-IP content of 94%, a further Vigreux distillation enriches the .beta.-IP content to 99%. This result corresponds, relative to amount of catalyst used and the time, to a yield of Y--9.4 liters .beta.-IP per kilogram catalyst and hour. This corresponds, relative to the educt solution used, to a yield of Y.sub.s-t =0.0376 l.sub..beta..sbsb.-IP /h/l.sub.solution.
Aside from the fact that the space-time yield is low and the accumulation of byproducts considerable, catalyst and distillation residue can not be readily separated in the homogeneous catalytic system used. Therefore, discarding is from time to time necessary since the temperature in the distillation bottom would otherwise rise too high. Even so, a "re-truing" of the temperature is required.
According to D6, the isomerization takes place in the liquid phase at temperatures around 200.degree. C. Silica gels with or without the addition of alkyl-substituted imidazolines of the following formula are used as catalyst. ##STR3##
Typical experimental conditions: 300 g .alpha.-IP and 25.7 g SiO.sub.2 are distilled for 52 h in the presence of refined steel; 230 g .beta.-IP (=76.6% yield) result with 99.0% purity. This result corresponds, relative to amount of catalyst used and the time, to a yield of Y=0.174 liters .beta.-IP per liter catalyst and hour.
However, the preparation of the organic bases is expensive and the space-time yield of the method low; with a characteristic value of Y=0.174 liters .beta.-IP/l catalyst per hour, even this method can not be converted to a technical scale. Relative to the volume of educt solution used the yield is Y.sub.s-t =0.0149 l.sub..beta..sbsb.-IP /h/l.sub.solution.
Moreover, the procedure described is unfavorable and the absolute production of .beta.-IP is low. The batchwise reaction and the performance of the isomerization reaction and pure distillation of the .beta.-IP in one step is especially disadvantageous. It can be demonstrated that the re-isomerization of .beta.-IP to .alpha.-IP occurs to a considerable extent on account of the high reaction temperature in the distillation apparatus.